Geographic Range

Green herons have a wide range in North America, but are generally found near wetlands. They occur as far north as southern Canada and as far south as northern South America. They are found throughout the eastern United States as far west as North Dakota and the Great Plains states, although some sedentary populations occur on the west coast. During the breeding season they are found primarily in the eastern United States, with some populations in the Pacific Northwest as well. Non-breeding individuals are found in Mexico and Central America, Texas, southern New Mexico and Arizona, and the Caribbean islands. Small vagrant populations winter in Hawaii and the United Kingdom. Some populations migrate and others are sedentary populations. Sedentary populations occur along the east and west coasts of the United States and Central America. Most populations in North America, however, are migratory. After breeding they disperse southwards, in mid-September. Spring migration occurs from March to April, an earlier arrival than most other herons.
(Davis and Kushlan, 1994; Hancock and Kushlan,1984) (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Habitat

Green herons live along forested water margins. Their general distribution is limited by the availability of wetlands. They frequent both salt and fresh water, showing great flexibility in habitat choice. Favored habitats are mangrove-lined shores and estuaries, and dense, woody vegetation fringing ponds, rivers and lakes (Hancock and Kushlan, 1984).

Physical Description

Green herons are small and stocky, with legs that are relatively short, compared to other herons. Body length ranges from 41 to 46 cm. Adults have a glossy greenish-black cap and back, wings that are black grading into green and/or blue on the edges, and underparts that are gray. The bill is dark with a long, sharp point and the legs are orange. Female adults tend to be smaller, with duller and lighter plumage than that seen in males, particularly in the breeding season.

Coloration of immature herons is different. The neck and chest are striped with white and shades of brown. The back is also brown with white and beige spots. The coloration of immature and adult birds is quite cryptic in dense vegetation.
(Hancock and Kushlan, 1984; Davis and Kushlan, 1994) (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Reproduction

Green herons are seasonally monogamous. Courtship displays are stereotyped. They begin with Flying Around displays resembling natural flight, but oriented to breeding sites with skow calls. From here, courtship becomes aggressive. Pursuit Flight, Circle Flight and Forward displays are used, where a rasping 'raah-raah' call exposes the red mouth-lining. Crooked-Neck Flight displays are more aggressive, where the neck is partially flexed, legs are dangled, and wingbeats are audible. Much like the Crooked-Neck Display, the Flap Flight Display shows the greatest intensity of flight displays. Here, the male lurches through the air with exaggerated flapping producing a whoom-whoom-whoom sound in a crooked-neck posture with crest, neck, and scapular feathers erect and often giving a roo-roo call before landing.

Nonaerial displays are interspersed with display flights. In the Snap Display, the male perches, then points body, head and neck down until bill tip is at or below the level of his feet and then snaps his mandibles together, producing a click while also erecting his feathers. The Stretch Display involves the male pointing his bill straight up, stretching his neck, and then bending it backwards until the head almost touches its back with interscapular plumes erect and fanned. In this posture, he sways his neck and head from side to side with crest, breast, and flank feathers sleeked back, eyes bulging, and iris possibly turning from yellow to deep orange while emitting an aaroo-aaroo sound.

Males perform this Stretch Display before a female is allowed to enter the eventual nest area. The female then performs a less intense Stretch silently after the male, which confirms the pair-bond. At this time, the male stops Flight and Snap displays. The pair then engages in mutual bill-snapping and feather nibbling, though those behaviors are reduced soon thereafter. Copulation occurs on the nest platform during the nest-building stage. It lasts about ten seconds with several hours between copulations. (Davis and Kushlan, 1994; Hancock and Kushlan, 1984) (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

After the last egg is laid, copulations cease and incubation persists for 19-21 days. A normal clutch is 2 to 4 eggs, laid in 2-day intervals. Fledging occurs when chicks are 16 to 17 days old, and independence is gained between 30 and 35 days.

Nesting takes place in forest and swamp patches, over water or in plants near water. Nesting pairs normally nest alone, but loose aggregations of mated pairs can form. Nest building is a cooperative effort, with the male participating more in protection versus actual construction. Nest placement can be from ground level to 20 meters, depending on plant height and thickness; branches in trees are favored. There is no attempt at nest sanitation, though chicks void over the edge of the nest once they're able to walk. (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Behavior

Green herons are shy birds so are rarely observed, although they may be quite common. They are active during the day. They have a characteristic slow, deliberate walk and in flight they have slow and steady wingbeats. They may also swim on occasion in pursuit of prey. Green herons are territorial and will aggressively defend both foraging and nesting territories from conspecifics. (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Communication and Perception

Green herons use their keen vision, hearing, and sense of touch to perceive their environment. They have especially acute vision that helps them to capture prey. Green herons have an elaborate set of calls and body postures that they use for communicating with other green herons. Examples are their elaborate courtship displays, warning calls when a predator is detected, and territorial displays. (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Food Habits

Green herons are carnivorous, mainly eating fish and invertebrates. They are opportunistic foragers with a broad prey base, depending on the availability of species present. They exploit superabundant food resources, such as breeding frogs. Their invertebrate diet includes: leeches, earthworms, dragonflies, damselflies, waterbugs, grasshoppers, and crayfish. Some of the many fish eaten are: minnows, sunfish, catfish, perch, eels, and, in urban areas, goldfish. Other vertebrates eaten are rodents, lizards, frogs, tadpoles, and snakes.

Their heavy bill enables them to capture large prey. Feeding can take place at any time, day or night. Typically, prey is captured with a darting stroke of the head and neck, lunging the body towards the victim and either grabbing or impaling the prey.

Among North American diurnal herons, green herons exhibit the fewest kinds of feeding behaviors. They are known to use 15 of the 36 heron feeding behaviors. The most common feeding technique is to stand in a crouched position, horizontal to the water surface, with neck and head retracted. They stand still for long periods of time before changing sites. Standing is often interspersed with slow walking in a crouched posture in the water or bordering vegetation. Herons use their feet to cause potential prey to move and then capture them. They may also dive from perches head first into deep water, becoming submerged, although this isn't a very efficient method. The greatest capture success is in shallowest water (0-10 cm) and the poorest success is in deeper water (20-30 cm).

Green herons are one of the few tool-using birds. They use a variety of baits and lures, such as crusts of bread, mayflies, and feathers. They then put the bait on the water surface and wait for prey to attack the bait. They stand motionless near the bait until a small fish or other animal approaches and then grab the prey. Success rates have been highest when live bait was used versus inanimate or dead bait. Juvenile herons are not as adept at baiting prey (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999). (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Predation

Snakes, crows, and common grackles are known to eat green heron eggs. Raccoons eat nestlings. Adult birds may be preyed on by large birds of prey. Green herons remain vigilant to protect themselves from predators. They also utter warning calls when predators approach. (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999)

Economic Importance for Humans: Positive

Economic Importance for Humans: Negative

Conservation Status

Human activities have had their effect on the heron. Historically, green herons were sometimes hunted for food. Fish hatcheries kill green herons to regulate their populations and prevent them eating their young fish. Pesticides have been found to accumulate in heron tissues, but no evidence of general reproductive failure has been observed, despite some localized effects to herons in highly polluted areas. Eggshell thinning has been recorded in comparison with pre-1947 egg samples, but it is not considered lethal. Increased recreational and industrial use of river channels leads to decreased use by green herons. Yet this has not led to decreased use of backwater habitats, for example, ponds. The main concern for the heron has been conservation and management of wetlands as a whole, versus the drainage and development that depletes habitat in which they live. No populations are considered threatened or endangered (Davis and Kushlan, 1994). (Davis and Kushlan, 1994)

Contributors

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal

active during the day, 2. lasting for one day.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

insectivore

An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males

marsh

marshes are wetland areas often dominated by grasses and reeds.

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

References

Davis, W., J. Kushlan. 1994. The Green Heron. The Birds of North America.

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This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.